Clay

Clay, soil particles the diameters of which are less than 0.005 millimetre; also a rock that is composed essentially of clay particles. Rock in this sense includes soils, ceramic clays, clay shales, mudstones, glacial clays (including great volumes of detrital and transported clays), and deep-sea clays (red clay, blue clay, and blue mud). These are all characterized by the presence of one or more clay minerals, together with varying amounts of organic and detrital materials, among which quartz is predominant. Clay materials are plastic when wet, and coherent when dry. Most clays are the result of weathering.

The clay Gay Head Cliffs in Martha’s Vineyard, Mass., U.S.

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No other earth material has so wide an importance or such extended uses as do the clays. They are used in a wide variety of industries. As soils, they provide the environment for almost all plant growth and hence for nearly all life on the Earth’s surface. They provide porosity, aeration, and water retention and are a reservoir of potassium oxide, calcium oxide, and even nitrogen.

The use of clay in pottery making antedates recorded human history, and pottery remains provide a record of past civilizations. As building materials, bricks (baked and as adobe) have been used in construction since earliest time. Impure clays may be used to make bricks, tile, and the cruder types of pottery, while kaolin, or china clay, is required for the finer grades of ceramic materials. Another major use of kaolin is as paper coating and filler; it gives the paper a gloss and increases the opacity. Refractory materials, including fire brick, chemical ware, and melting pots for glass, also make use of kaolin together with other materials that increase resistance to heat. Certain clays known as fuller’s earth have long been used in wool scouring. In rubber compounding, the addition of clay increases resistance to wear and helps eliminate molding troubles.

Clay materials have a wide variety of uses in engineering. Earth dams are made impermeable to water by adding suitable clay materials to porous soil; water loss in canals may be reduced by adding clay. The essential raw materials of portland cement are limestone and clays, commonly impure. After acid treatment, clays have been used as water softeners; the clay removes calcium and magnesium from the solution and substitutes sodium. A major use of clay is as drilling mud—i.e., heavy suspension consisting of chemical additives and weighting materials, along with clays, employed in rotary drilling.

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Clay is a natural material made up of tiny particles of rock. When clay is mixed with enough water, it feels like soft, gluey mud. Unlike plain mud, however, clay holds its shape. Clay can be pinched, rolled, cut, or built up in layers to form shapes of all kinds.

Soil particles that come from rock and have diameters smaller than 0.0002 inch (0.005 millimeter) are collectively called clay. Particles of clay, when mixed with the proper amount of water, cling together in a soft, sticky mass. A lump of wet clay is said to be plastic because it can be squeezed or pressed into any desired shape. The synthetic materials commonly called plastics take their name from the same property, but they are softened by heat rather than water. When dried, a molded clay object holds its form. If it is wetted it will soften again; however, if it is fired, or baked at a high temperature, the object becomes as hard as stone and will no longer soften in water. Thousands of years ago, during the Neolithic stage of civilization, people learned how to mold and bake clay to form bricks and pottery (see civilization). The ancient Sumerians even wrote on wet clay tablets (see Babylonia and Assyria).

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